A synthetic resin web which is produced by a slit nozzle is generally smoothed and used as a packaging material. The plastic films for this purpose should have a high degree of transparency. The transparency, however, is dependent on the ratio of the crystallization speed to the speed at which the web is cooled down. A high cooling rate, as a rule, produces plastic film of relatively high transparency. Thus it is desirable to cool such plastic webs as rapidly as possible and with as high a temperature gradient or heat extraction rate as is possible.
In the standard practices, the synthetic resin web emerging from the slit nozzle or slot die is passed through a smoothing and cooling gap which can be defined on the one hand by a cooled smoothing roller or roll and, on the other hand, by a portion of the periphery of a smoothing roll over which a cooled endless belt is passed. The endless belt passes around rerouting rollers at its opposite ends and each of these rerouting rollers may be a cooled roller.
In the use of an endless belt in the manner described it is found that after the plastic web has been contacted by the upstream cooling roller of the belt, there is a tendency for the plastic web to pick up heat rapidly although it lies in contact with the belt between the rerouting rollers.
In other words, the plastic web tends to be reheated during contact with the endless belt and before it again passes into contact with the second cooling roller of the belt. The smoothing effect is largely unsatisfactory in such causes. The surface of the plastic web is generally found to be detrimentally effected by this property of the endless belt system.
Furthermore, when the endless belt, as is not uncommon, has some irregularity like for example a weld seam, corresponding undesirable markings are found in the plastic web. Since the cooling effect with this system is less than satisfactory and marking can occur, the system has been found to be undesirable for producing plastic film of high transparency.